The invention herein resides in the art of brake control systems and more particularly relates to such systems for aircraft. The invention includes a circuit for controlling the rate of deceleration of an aircraft and accordingly for controlling the applied brake pressure when the aircraft speed is below a particular level. It will be understood that while the invention herein is described with respect to an aircraft, particularly one having four braked wheels, the invention is applicable to any vehicle having any number of braked wheels.
Heretofore in the aircraft industry, pilots have often experienced a "grabbiness" in brakes when the aircraft runway speed has dropped below a particular level. This grabbiness is usually a result of the brake mix or material composition of the vehicle brakes themselves. At high runway speeds, during the initial braking effort, no grabbing is experienced. This is because the brakes are, at this point in time, operating at a very high temperature. However, as the aircraft begins to slow down and the brakes begin to cool, they become tacky. This tackiness is believed to result from the fact that during initial brake application there is an actual sintering of the brake material and as the vehicle slows down and the brakes begin to cool, the sintered brake material makes the braking surfaces tacky, causing the brakes to grab. The result is passenger discomfort and control difficulties from the resultant strut vibrations.
It will be understood to those skilled in the art that the physical design and structure of the aircraft strut may tend either to amplify or dampen the vibrations resulting from brake grab. However, costly and complex strut designs are often required to alleviate the strut vibrations and, if the choice is made in favor of a lighter weight design, the physical structure of the strut may, indeed, accentuate the vibrations.
It should also be appreciated that brake grab may be reduced or substantially eliminated by appropriate selection of brake mix or composition. However, a trade off generally has to be made between brake wear and reduction in brake grab. Softer brakes, being substantially free of brake grab, generally have high wear rate and are thus costly with respect to the periods of required replacement. Of course, carbon brakes are not characteristically grabby, but are too expensive for many applications.